Switch discovery protocol for a distributed fabric system

ABSTRACT

A distributed fabric system comprises a plurality of independent network elements interconnected by inter-switch links and assigned to a same group. Each network element includes one or more switching chips, a processor, and memory storing program code that is executed by the processor. The program code of each network element includes a switch discovery protocol (SDP) module. The SDP module of each network element, when executed, periodically multicasts SDP data units (SDPDUs) using one of a plurality of transmission rates. The plurality of transmission rates includes a fast transmission rate and a slow transmission rate. The transmission rate used by the SDP module of each network element is the fast transmission rate until the SDP module of that network element determines a criterion is met, in response to which the transmission rate used by the SDP module of that network element changes to the slow transmission rate.

RELATED APPLICATION

This application is a continuation application claiming the benefit ofthe filing date of U.S. patent application Ser. No. 13/364,947, filedFeb. 2, 2012, and titled “Switch Discovery Protocol for a DistributedFabric System,” the contents of which are incorporated by referenceherein in their entirety.

FIELD OF THE INVENTION

The invention relates generally to data centers and data processing.More particularly, the invention relates to a protocol for detectingwhen switches join and leave a distributed fabric system and fordetermining a best available path for communication between switches inthe system.

BACKGROUND

Data centers are generally centralized facilities that provide Internetand intranet services needed to support businesses and organizations. Atypical data center can house various types of electronic equipment,such as computers, servers (e.g., email servers, proxy servers, and DNSservers), switches, routers, data storage devices, and other associatedcomponents. A given data center can have hundreds or thousands ofswitches interconnected in a distributed fashion. Often, multipleindividual switches are grouped into a distributed system. Any one ofthese switches can leave or join the distributed fabric at any giventime, with each such event being a potential disruption to the properoperation of the distributed fabric system.

SUMMARY

In one aspect, the invention features a method for discovering networkelements assigned to a same group in a distributed fabric system. Themethod comprises periodically multicasting, by each network element of aplurality of network elements, switch discovery protocol data units(SDPDUs) using one of a plurality of transmission rates. The pluralityof transmission rates including a fast transmission rate and a slowtransmission rate. One or more of the network elements detects stabilityin the distributed fabric system and changes the transmission rate fromthe fast transmission rate to the slow transmission rate in response tothe detected stability.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of this invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which like numerals indicate likestructural elements and features in various figures. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention.

FIG. 1 is an embodiment of a networking environment including a datacenter with a plurality of network elements, a server, and a managementstation.

FIG. 2 is a block diagram of an embodiment of the data center includinga master (controller) switch, a back-up switch, and a plurality offollower switch.

FIG. 3 is a functional block diagram of an embodiment of a networkelement including a processor in communication with memory, and alayered software stack stored in the memory.

FIG. 4A is a block diagram of the layered software in a master switchand various communication channels between layers of the software stack.

FIG. 4B is a block diagram of the layered software in a follower switchand various communication channels between layers of the software stack.

FIG. 5 is a diagram of an embodiment of a Protocol TLV(type-length-value).

FIG. 6 is a diagram of an embodiment of a Group ID (GID) TLV.

FIG. 7 is a diagram of an embodiment of a Switch-Info (SI) TLV.

FIG. 8 is a diagram of an embodiment of a Switch-Members (SM) TLV.

FIG. 9 is a diagram of an embodiment of a transmit state machine (TSM)for the switch discovery protocol.

FIG. 10 is a diagram of an embodiment of a receive state machine (RSM)for the switch discovery protocol.

DETAILED DESCRIPTION

Distributed fabric systems described herein include a plurality ofinterconnected independent network elements. Each of these networkelements includes one or more switching chips for routing packetsthroughout the distributed fabric. Hereafter, such network elements mayinterchangeably be referred to as switches. These network elementscommunicate with each other in accordance with certain protocols. One ofthe protocols is a switch discovery protocol (SDP), by which the networkelements detect when a network element joins or leaves the distributedfabric system. The rapid detection of changes to the membership of thedistributed fabric system is important to the proper operation of thedistributed fabric system. In addition, the SDP contributes to theprocess of selecting paths for packets to travel through the distributedfabric system from one network element to another, and, in particular,to the process of selecting a new path quickly when an old path becomesunavailable.

FIG. 1 shows an embodiment of a networking environment 2 including adata center 10 in communication with a management station 4 and a server6 over a network 8. Embodiments of the network 8 include, but are notlimited to, local-area networks (LAN), metro-area networks (MAN), andwide-area networks (WAN), such as the Internet or World Wide Web. In oneembodiment, the network 8 is configured as an Layer 2 (L2) VLAN. Thedata center 10 is generally a facility that houses various computers,routers, switches, and other associated equipment in support ofapplications and data that are integral to the operation of a business,organization, or other entities.

The data center 10 includes a plurality of network elements 14 incommunication over inter-switch links (ISLs) 16. The network elements 14are independent (standalone) packet-based switches, configured togetherto form a single distributed fabric system, each designated as a memberof a particular group (or cluster). Each group has a master (orcontroller) network element, one or more standby or back-up networkelements, and one or more follower network elements, as described inmore detail in connection with FIG. 2. The data center 10 can have morethan one group, although each network element can be the member of onegroup only. Members of the same group share the same group ID (GID).Embodiments of the network elements 14 include, but are not limited to,core switches, access switches, fabric cards, line cards, and managementmodules in a physical chassis switch. Although only five networkelements 14 are shown, the number of network elements in the distributedfabric system can range in the hundreds and thousands.

The data center 10 may be embodied at a single site or distributed amongmultiple sites. Although shown outside of the data center 10, either (orboth) of the management station 4 and server 6 may be considered part ofthe data center 10. In the data center 10, the functionality occurs onthree planes: a management plane, a control plane, and a data plane. Themanagement of the group, such as configuration management, runtimeconfiguration management, presentation of information (show anddisplay), graph generation, and handling SNMP requests, occurs on themanagement plane. The control plane is associated with those functionsinvolving network signaling and control. The data plane manages dataflow. In the data center 10, the functionality of the management planeis centrally implemented at a master network element, as describedfurther herein. The functionality of the control plane may beimplemented predominately at the server 6 or be distributed among thenetwork elements. In general, the functionality of the data plane isdistributed among the network elements 14.

The management station 4 provides a centralized point of administrationfor managing and controlling the networked switches 14 of thedistributed fabric system. Through the management station 4, a user ornetwork administrator of the data center 10 communicates with the masternetwork element in order to manage the group, with conceivably thousandsof network elements, from a single location. A graphical user interface(GUI) application executing on the management station 4 can serve toprovide the network administrator with a view of the entire networktopology of the distributed fabric system. An example of such a GUIapplication is Blade Harmony Manager® provided by IBM Corporation ofArmonk, N.Y.

In addition, the management station 4 can connect directly(point-to-point) or indirectly to the master network element 14 of thedata center 10 over one of a variety of connections, such as standardtelephone lines, digital subscriber line (DSL), asynchronous DSL, LAN orWAN links (e.g., T1, T3), broadband connections (Frame Relay, ATM), andwireless connections (e.g., 802.11(a), 802.11(b), 802.11(g), 802.11(n)).Using a network protocol, such as Telnet or SNMP (Simple NetworkManagement Protocol), the management station 4 can access a command-lineinterface (CLI) of the given network element 14.

In general, the server 6 is a computer (or group of computers) thatprovides one or more services to the data center 10, examples of whichinclude, but are not limited to, email servers, proxy servers, DNSservers, and a control server running the control plane of thedistributed fabric system. To support the control plane functionality ofan entire network element cluster, the server 6 is configured withsufficient processing power (e.g., with multiple processor cores).

FIG. 2 shows an embodiment of the data center 10 with the plurality ofnetwork elements 14 including a master (controller) switch 14-1, abackup (standby) switch 14-2, and a plurality of follower switches 14-3,14-4, 14-N. In general, one of the network elements is chosen as themaster switch 14-1, another is designated as the backup switch 14-2, andall other switches are followers. The master switch 14-1 is the controlcenter for the entire distributed fabric system and the follower switchis any network element under the control of the master switch 14-1, themaster switch 14-1 sending and receiving control plane and data planepackets to and from the follower switches 14-3, 14-4, 14-N. Under normaloperation of the distributed fabric system, the backup switch 14-2operates like a follower switch, except that the backup switch 14-2assumes the role of master switch in the event the master switch fails.Unless specifically excluded, a reference hereafter to a follower switchincludes the backup switch.

The master switch 14-1 and backup switch 14-2 are each in communicationwith each of the follower switches 14-3, 14-4, 14-N over ISLs 16. Otherinterconnection configurations can be employed, such as daisy chain,full mesh, star, and stacked, without departing from the principlesdescribed herein. In one embodiment, the ISLs 16 over which the networkelements 14 communicate are 10 Gb Ethernet links (the network elements14 communicating according to the IEEE 802.Qgb standard).

Applications in such a distributed fabric system preferably have threemodes: a master mode, a backup mode, and a member mode. Depending uponthe role of a given network element, applications running on thatnetwork element run in the corresponding mode. For example, applicationsrunning on the master switch 14-1 run in the master mode. Eachapplication can take a different approach and, thus, take differentresponsibilities in the different modes. Example implementations ofthese applications include a purely centralized approach, a fullydistributed approach, or a combination of centralized and distributedapproaches. Applications running on a network element have a global viewof all the data ports on all network elements in the distributed fabricsystem.

FIG. 3 shows a simplified embodiment of a network element 14 including aprocessor 20 in communication with memory 22, and layered software 24stored in the memory 22. The layered software 24 includes a set ofsoftware components common to each of the network elements 14. In brief,the set of software components includes protocols for grouping themultiple network elements 14 together to form a single large switch. Byimplementing the protocols provided by this set of software components,referred to herein as M-DFP or Management Distributed Fabric Protocol,the group of network elements can be connected to form a stacked switch,a virtual switch, or a distributed chassis switch. This set of softwarecomponents can also serve to implement a physical chassis switch. Ingeneral, the M-DFP software components reside in the software stack 24between those applications on a network element and the SDK (softwaredevelopment kit) on a system. An SDK includes runtime tools, such as theLinux kernel, development tools, software libraries, and frameworks.

The layered software stack 24 includes a path selection layer 26, aswitch discovery protocol (SDP) module 28, an EL2T (Ethernet-based L2Transport) layer 30, an RPC (Remote Procedure Call) module 32, a portmapping/macros module 34, a DC-stacking module 36, DC APIs (applicationprogram interface) 38, a switch attach/detach module 40, a CP (checkpoint) module 42, and a TFTP (Trivial File Transfer Protocol) module 44.The communications required to implement M-DFP among the networkelements 14 can run on top of standard Ethernet links, a fabricconnection, or any proprietary bus.

In brief overview, the path selection layer (PSL) 26 facilitatesCPU-to-CPU communications in support of the SDP and EL2T modules 28, 30,and includes a driver interface to both socket and data ports. The SDPmodule 28 and the PSL 26 cooperate to determine the egress port by whicha packet is sent from the local network element to a remote networkelement. As used herein, the terms “local” and “remote” are withreference to the network element being described.

To determine the egress port, the SDP module 28 maintains the linkstates for each possible ISL 16. In addition, the SDP module maintains ahealth record for each remote switch and for each possible path/port.The health record can keep track of the following information: thehealth state of the path, and the number of SDPDUs received during acurrent interval. The master switch maintains the health records for allthe switches (master and followers). A follower switch maintains therecords for the master switch only (in general, follower switches do notcommunicate with other follower switches).

For normal path selection, the current path is favored if the path hasan “up” link state and is healthy. If the current path for a remotenetwork element is no longer healthy or available, the health recordsserve to choose the next best path. Example criteria for selecting thebest path is to find the path with an “up” link state that has had themost SDPDUs received during the most recent period, and which no othermodules have reported to be unhealthy. After a new path for a remotenetwork element is determined, the SDP module 28 notifies the PSL 26 ofthis new path. After a predefined interval, the health records are resetto ensure the information in the health records is current.

The SDP module 28 discovers when network elements join and leave thegroup, referred to as switch-found and switch-gone events, respectively.Detecting the departure of a network element can be achieved using anage-out mechanism. Link-down events on the ISLs 16 can also triggerswitch-gone detection under some conditions. The SDP module 28 reportsswitch-found (JOIN_STACK) and switch-gone (LEAVE_STACK) events to theDC-stacking module 36 on the same network element for furtherprocessing. Other functions of the SDP module 28 are to check the healthof ISLs 16 for all possible paths between the local network element andother remote network elements, and to provide a priority-basedmaster-election mechanism.

The EL2T layer 30 provides a simple L2 transport protocol to facilitatecommunications by the upper layer protocols above the EL2T layer 30. Inone embodiment, these upper layer protocols include the RPC module 32,the DC-stacking module 36, the CP module 42, the TFTP module 44, and allapplications on the network element 14.

The RPC module 32 provides an RPC mechanism that is based on the EL2Tlayer 30, and used by DC-API layer 38 on the master switch 14-1 tocommunicate with a remote network element.

The port mapping/macros module 34 provides applications on the top ofthe layered software with a mapping from a global CLI port to a physicaldevice and a port. In cooperation with the DC-stacking module 36 and theSDP module 28, the port mapping/macros module 34 maintains the mapping.

The DC-stacking module 36 forms a “stack” of the network elements in thesame group, coordinating the network elements such that they cooperateas a single switch. The DC-stacking module 36 of all network elements inthe same group communicate with each other using the EL2T module 30 forinformation exchange and for stack formation. In addition, theDC-stacking module 36 on different network elements work together tomake sure the master switch 14-1 has up-to-date information for existingnetwork elements (through HOST-UPDATE events). A HOST-UPDATE event ispassed to the DC-stacking module 36 to provide an information updatewhenever the switch information for a given network element has changedand the DC-stacking module 36 has already received a JOIN-STACK eventfor that given network element.

Through the DC-API layer 38, applications running on the network element14 can make program calls to the hardware switching chips of the networkelement, either to retrieve information from the chips or to set someparameters on the chips. These chips may reside either on the localnetwork element or on a remote network element.

The switch attach/detach module 40 notifies applications on the networkelement of changes on the network element, thus providing applicationsat the top of the layered software stack 24 with a global view of alldata ports on all network elements in the group.

The CP module 42 assists applications running on the master switch 14-1to synchronize each relevant database and states with the backup switch14-2 in preparation for a backup-to-master failover.

The TFTP module 44 provides a transport layer on top of the EL2T layer30 to assist the DC-stacking module 36 and applications to push either aconfiguration or a firmware image from the master switch 14-1 to anyfollower switch 14-3, 14-4, 14-N.

FIG. 4A and FIG. 4B show software stacks 24 in a master switch 14-1 andin a follower switch 14-3 (as a representative example of the followerswitches), respectively. Here, each software stack 24 includes anapplication layer 50 with various applications 54, examples of whichinclude a config application, a CLI application, and a syslogsapplication. Double-ended arrows 52 represent control flows betweencomponents in the software stack 24.

In the layered software stack 24, the SDP module 28 is disposed betweenthe DC-stacking module 36 above and the path selection layer 26 below.The SDP module 28 includes a switch discovery protocol (SDP), a membertracking layer (MTL), and a Path Health Maintenance (PHM) component. SDPis a multicast protocol, running in a common L2 VLAN, used for thediscovery of switches in the distributed fabric system. After a switchreceives a packet for SDP, related switch information is passed into theMTL for maintenance of membership.

The MTL is a database layer of the SDP module 28 for tracking thecurrent network element members in the same group and for maintainingswitch information for all such members. The switch information for eachnetwork element includes: the switch number, the MAC address of theswitch, switch information (SI) and switch member (SM) sequence numbers,and a time at which the last SDPDU is received from a remote networkelement. Any changes to the switch information is reported to MTL fortracking. When an ISL 16 goes down, the switch information learned overthat link is cleared in the MTL. To help detect a switch-gone event, theMTL implements an age-out mechanism, using timers to “age out” a remotenetwork element if no SDPDU is received from that network element for aspecified duration.

The MTL also elects the master switch of a group based on switchpriorities (carried in the SDPDUs multicast by the network elements).After the election, the elected master switch reports the switch memberinformation to the DC-stacking module 36 of the master switch. Inaddition, the MTL of the master switch passes a message to theDC-stacking module 36 to notify of any change in switch membership inthe group, whether resulting from a newly discovered network element orfrom detecting the departure of a network element.

The PHM component of the SDP module 28 maintains the health states ofall possible paths between the local network element and all otherremote network elements. When an SDPDU is received from a networkelement, the health states for that network element are also updated inthe MTL. As previously described, the EL2T 30 and PSL 26 use this healthinformation to determine the path or port used for communication betweenthe local network element and a remote network element.

After booting up, a local network element periodically transmits “SDPHello” frames (i.e., PDUs) over the ISLs 16 to announce its presence toall other peers in the same VLAN. In response to receiving an “SDPHello” PDU, each network element collects and maintains the switchinformation in its local database (i.e., the MTL). After the networkelements collect enough switch information, the SDP module 28 of eachnetwork element elects a master switch based on switch priorities (allnetwork elements elect the same master switch). The SDP module 28 ofeach network element passes its collected switch information in anappropriate manner to the DC-stacking module 36 on that same networkelement; if the network element is the master switch for the group, theswitch information is used for stack coordination and formation.

Packets issued by the SDP module 28 are called switch data protocol dataunits (SDPDUs). Generally, an SDPDU is a TLV-based PDU (protocol dataunit). SDPDUs use four types of TLVs: a Protocol TLV, a Group-ID (GID)TLV, Switch-Info (SI) TLV, and a Switch-Members (SM) TLV. FIG. 5 showsan embodiment of a Protocol TLV 60 including a type field 62, a lengthfield 64, an OUI (Organization Unique Identifier) field 66, a subtypefield 68, and a protocol ID field 70. The Protocol TLV 60 serves toidentify the frame type of a PDU. The type field 62 identifies the startof a TLV (e.g., value=127); the length field 64 indicates the bit lengthof the Protocol TLV; the OUI field 66 holds a proprietary company ID;the subtype field 68 holds a value (here, equal to 1) that identifiesthe type of the TLV (here, denoting a Protocol TLV); and the protocol IDfield 70 identifies the protocol (e.g., Ethernet).

FIG. 6 shows an embodiment of the GID TLV 80, which is used for groupidentification. Multiple network elements in the same VLAN may belong todifferent groups and, thus, to different virtual switches (or virtualchasses). The GID TLV 80 includes a type field 82, a length field 84, anOUI field 86, a subtype field 88, and a group ID field 90. The value inthe type field 82 identifies the start of a TLV (e.g., value=127); thelength field 84 indicates the bit length of the TLV; the OUI field 86holds a company ID; the subtype field 88 holds a value of 2 to indicatethis TLV is a GID TLV 80; and the group ID 90 identifies the group ofnetwork elements to which the transmitting network element belongs.

FIG. 7 shows an embodiment of the Switch-Info (SI) TLV 100. The SI TLV100 is used to advertise the switch information of the local networkelement and includes a type field 102, a length field 104, an OUI field106, a subtype field 108, an SI sequence number field 110, an SIsequence number acknowledged field 112, an SM sequence numberacknowledged field 114, and a field 116 for carrying switch information.The roles of the type field 102, length field 104, and OUI field 106 aresimilar to those of the Protocol TLV 60 and GID TLV 80. A value of 3 inthe subtype field 108 identifies the type of this TLV as the SI TLV 100.

The SI sequence number field 110 holds the sequence number of thisparticular TLV. The sequence number increments by one in response to anychange in the switch information in the TLV (exceptions to this beingthat changes to the acknowledged SI or SM sequence numbers in the fields112, 114 do not result in incrementing the SI sequence number). In oneembodiment, the sequence number 0 is used to denote an invalid SI; thatusually indicates no SI received of updated before. An SDPDU with avalid SI should never use the SI sequence number 0. The SI sequencenumber acknowledged field 112 holds the last sequence number of aSwitch-Info TLV sent by the local network element and acknowledged bythe master switch. This value is set to 0 if sent by the master switch.The master does not use the SI sequence number acknowledged field 112 toacknowledge the SI received from a member. Instead, the master uses theSI sequence number field 172 for acknowledgements. Usually the systemhas more than one member. The SI sequence number acknowledged field 112is used by a member switch to acknowledge the SI received from themaster. The SM sequence number acknowledged field 114 holds the lastsequence number of a Switch-Members TLV received from the master switch.This field 114 is used by member switches only and is set to 0 if sentby the master switch.

The switch information field 116 includes fields for a switch number118, switch priority 120, CE/FE (control element/forwarding element)type 122, switch type 124, bay ID 126, number of ports 128, MAC address130, UUID 132, ISL bitmask 134, and flags 136. The switch number field118 holds a switch number that is set by the transmitting networkelement. The switch priority field 120 holds a switch priority of thetransmitting network element, which can be used to elect the masterswitch. The CE/FE type field 122 identifies the type of the switch as acontrol element (CE), forwarding element (FE), or both. The switch type124 identifies the switch type of the transmitting network element.Switch type is the type of the switch; the type determines the portconfiguration and port type of a switch. CE denotes the control element;FE denotes the forwarding elements; only a CE can be the master or thebackup controller in a distributed system. The bay ID 126 can identifythe physical location of the network element within a physical chassis.The number of ports 128 identifies the number of ports, the MAC address130 holds the MAC address, the UUID field 132 holds the UUID, and theISL bitmask field 134 holds the bitmask of the local ISL ports, all withrespect to the transmitting network element.

The flags field 136 has a one-bit field 138 for the transmitting networkelement to signify that it is the master switch and a one-bit field 140for the transmitting network element to signify that it is the backupswitch.

FIG. 8 shows an embodiment of a Switch-Members (SM) TLV 150 (transmittedby the master switch only). The master switch uses the SM TLV 150 tonotify the follower switches of all the member information that themaster switch has collected for the same group of network elements. TheSM TLV 150 includes a type field 152, a length field 154, an OUI field156, a subtype field 158, an SM sequence number field 160, a masterswitch number field 162, a backup switch number field 164, a field 166identifying a number of entries carried by the SM TLV, and a field 168for carrying the membership information. Each entry in the membershipinformation field 168 includes a switch number field 170, a SI sequencenumber field 172, and a switch MAC address 174.

The roles of the type field 152, length field 154, and OUI field 156 aresimilar to those of the Protocol TLV 60, GID TLV 80, and SI TLV 100. Avalue of 4 in the subtype field 158 identifies the type of the TLV asthe SM TLV 150. The SM sequence number field 160 holds the sequencenumber of this particular SM TLV. The sequence number increments by onein response to any change in the SM TLV. The Sequence number 0 isdenotes an invalid SM TLV; either not received or not updated. An SDPDUwith a valid SM does not use sequence number 0 in the SM sequence numberfield. The master switch number field 162 holds the switch number of theCE-master switch. The value is equal to 0 if the master switch is notassigned yet. The backup switch number field 164 holds the switch numberof the backup switch. The value is equal to 0 if the backup switch isnot yet assigned yet. Field 166 holds the number (N) of entries carriedby the SM TLV 150. Each switch member entry includes the switch numberof the member (0, is not yet assigned), the SI sequence number of thelast Switch-Info TLV received from that member switch, and the switchMAC address of that member switch.

All network elements multicast SDPDUs periodically. For the masterswitch, an SDPDU includes the following TLVs: a Protocol TLV 60, a GIDTLV 80, a Switch-Info TLV 100, and Switch-Members TLV 150. For allfollower switches, each SDPDU includes: a Protocol TLV 60, GID TLV 80,and a switch-info TLV 100. The packet format of an SDPDU includes an ECP(Edge Control Protocol) header (e.g., Ethertype), followed by theProtocol TLV 60, the GID TLV 80, the SI TLV 100, and, in the case of themaster switch, by the SM TLV 150. ECP is a protocol defined in 802.1Qbgfor PDU transport needed by the VDP protocol.

In the processing of SDPDUs, the SDP module 28 has two state machines: aTransmit State Machine (TSM) 200 and a Receive State Machine (RSM) 250.FIG. 9 shows an embodiment of the TSM 200 for the SDP. The TSM 200controls a process by which the network elements issue SDPDUs. Thetransmission of SDPDUs can occur in two different modes: a fast-transmitmode and a slow-transmit mode; the fast-transmit mode has a fastertransmission rate than the slow-transmit mode. An objective of the TSM200 is to reduce CPU overhead used to run the SDP when the distributedfabric system (i.e., all the network elements and related ISLs) havebecome stable by entering the slow-transmit mode, and to speed up switchdiscovery in response to any detected change by entering thefast-transmit mode. The retransmission rate of SDPDUs slows down afterthe master switch and follower switches have cully exchanged the currentswitch information of all members in the group. The retransmission ratecan also slow down whenever the master switch or follower switches hasno further changes for its transmitted SDPDUs.

The TSM 200 enters the fast-transmit mode when: (1) the network elementstarts up, (2) a TLV changes (e.g., master, backup, or follower changes;or, switch information ages out), (3) the master has set theacknowledged SI sequence number in the Switch-Members TLV to 0 (amechanism used by the master switch to request a response from a remotenetwork element immediately), (4) the current multicast path fails(required for SMAC learning), and (5) when a master failover occurs. Forexample, in the event of a master switch failure, the backup switchassumes the role of the new master switch. The other switches eventuallydetect this change and discard all the switch information learned fromthe old master switch (gained through the Switch-Members TLV 150). This“TLV changes” event prompts all switches to enter the fast-transmitmode. As another example, a change in a remote Switch-Info TLV 100triggers a change of the Switch-Members TLV on the master switch. Thus,the joining of a new switch prompts the master switch to enter thefast-transmit mode. The entering of the master switch into thefast-transmit mode, however, does not trigger a follower switch to enterthe fast-transmit mode.

The TSM 200 includes an idle state 202, a set-TLV-to-0 state 204, afast-transmit state 206, a transmit state 208, a slow-transmit state210, a move-to-slow-transmit state 212, a set-TLV-to-1 state 214, and amove-to-fast-transmit state 216. Three states are considered stablestates: the idle state 202; the fast-transmit state 206, in which theSDPDU is transmitted at the fast transmission rate; and theslow-transmit state 210, in which the SDPDU is transmitted at a slowtransmission rate. One transmit timer with two settings can be used toimplement two transmission rates: one setting for the fast-transmit mode(e.g., 1-second intervals), and the second setting for the slow-transmitmode (e.g., 30-second intervals).

Transitions from state-to-state are influenced by the values of certainvariables maintained by the TSM 200. In one embodiment, the variablesare called “RETRIAL” and “LOCAL-FAST-XMIT”. The RETRIAL variablecontains the number of transmittals for the last SDPDU (in other words,consecutive transmissions of an unchanged SDPDU, which signifiesconverging stability). The LOCAL-FAST-XMIT variable is a Boolean fordenoting if the TSM 200 is currently in the fast-transmit mode. Thevalues of certain parameters also affect the operation of the TSM 200.In one embodiment, the parameters are called “FAST-XMIT-INTERVAL”,“SLOW-XMIT-INTERVAL”, and “SLOW-XMIT-THRESHOLD”. The value of theFAST-XMIT-INTERVAL parameter controls the transmit timer in thefast-transmit mode. A default value sets the transmit timer to 1 second.If the value is equal to 0, no SDPDU transmission occurs. The value ofthe SLOW-XMIT-INTERVAL parameter controls the transmit timer in theslow-transmit mode. A default value sets the transmit timer to 30seconds. If the value is equal to 0, no SDPDU transmission occurs. TheSLOW-XMIT-THRESHOLD parameter controls when the TSM transitions to theslow transmit mode, namely, when the RETRIAL variable exceeds thisthreshold value, signifying a given number of consecutive transmissionsof the same SDPDU. For the master switch, the default threshold is 10;for follower switches, the default threshold is five.

Six events can trigger a state transition in the TSM 200: the networkelement starts up; the network element shuts down; a TLV changes; atimer expires; the master switch requests a move to the fast-transmitmode (accordingly, this event can occur only on a follower switch); anda “TLV agreed” event (triggered by the RSM 250). The TSM 200 enters theslow-transmit mode when a TLV agreed event occurs. A TLV agreed eventsignifies: 1) for the master switch, that the local sequence numbers(both SI and SM) have been acknowledged by all the follower switches,and the master switch encounters no changes in the remote SI sequencenumbers received from all follower switches; and 2) for each followerswitch, that its local SI sequence number has been acknowledged by themaster switch, and the follower switch encounters no change in thesequence numbers (both SI and SM) received from the master switch. For aTLV agreed event to be valid, the sequence numbers cannot be equal to 0.

A network element enters the idle state 202 when it initializes and whenit shuts down. While in the idle state 202, the RETRIAL variable is setequal to 0 and the LOCAL-FAST-XMIT variable is set equal to 1. If, whilein the idle state 202, a TLV changes, the TSM 200 transitions to theSet-TLV-to-0 state 204. A TLV change means that the network element ismaking a modification to any one of the TLVs in its next SDPDU. In theSet-TLV-to-0 state 204, the TSM 200 makes the TLV change, and uponcompletion (UCT) returns to the idle state 202. When the TLVs are stable(i.e., no TLV changes), the TSM 200 transitions from the idle state 202to the fast-transmit state 206.

In response to the transition to the fast-transmit state 206, thetransmit timer stops and restarts. When the transmit timer expires, theTSM transitions to the transmit state 208. In the transmit state 208,the RETRIAL variable is incremented. If the resulting value of theRETRIAL variable exceeds the SLOW-XMIT-THRESHOLD value, theLOCAL-FAST-XMIT variable is set equal to 0, and the network elementtransmits the SDPDU. If, after transmitting the SDPDU, the value of theLOCAL-FAST-TRANSMIT variable is equal to one, the TSM 200 returns to thefast transmit state 206; if equal to zero, the TSM 200 transitions tothe slow transmit state 210. When the value of the RETRIAL variableexceeds the SLOW-XMIT-THRESHOLD value (signifying the number oftransmittals of an unchanged SDPDU has exceeded a specified threshold),the network element enters the slow-transmit mode, to slow down thetransmission of SDPDUs because all peer network elements have no morechanges to make.

If, while in the fast transmit state 206, a TLV agreed event occursbefore the transmit timer expires, the TSM 200 transitions to themove-to-slow transmit state 212; or if a TLV changes event occurs beforethe transmit timer expires, the TSM 200 transitions to the set-TLV-to-1state 214. In the move-to-slow transmit state 212, the LOCAL-FAST-XMITvariable is set equal to 0, and the TSM 200 transitions to theslow-transmit state 210 upon completion (UCT). In the set-TLV-to-1 state214, the TLV change is made, the RETRIAL variable is set equal to 0 andthe LOCAL-FAST-XMIT variable is set equal to 1. Upon completion of thesesettings, the TSM 200 returns to the fast transmit state 206.

In response to the transition to the slow transmit state 210, thetransmit timer stops and restarts, and runs according to the slowtransmission rate. When the transmit timer expires, the TSM 200transitions to the transmit state 208. If, while in the slow transmitstate 210, the master switch requests a faster transmit rate before thetransmit timer expires, the TSM 200 transitions to the move-to-fasttransmit state 216; or if a TLV changes event occurs before the transmittimer expires, the TSM 200 transitions to the set-TLV-to-1 state 214. Inthe move-to-fast transmit state 216, the RETRIAL variable is reset (setequal to 0) and the LOCAL-FAST-XMIT variable is set equal to 1, and theTSM 200 transitions to the Transmit state 208 upon completion (UCT).Again, in the set-TLV-to-1 state 214, the TLV is set, the RETRIALvariable is reset equal to 0, the LOCAL-FAST-XMIT variable is set equalto 1, and, upon the completion of these settings, the TSM 200 returns tothe fast-transmit state 206.

In general, the SDP is used primarily to discover a new switch, and, aspreviously described, the MTL implements an age out mechanism to detectwhen a switch leaves the distributed fabric system. The age-outmechanism is implemented in the master switch and in the other switches.The master switch runs a timer to “age out” a SI sequence number of aremote switch in its Switch-Members TLV, if the related Switch-Info TLVhas not been received for a specified period (e.g., 100 seconds). Whenthis SI sequence number ages out, the master switch triggers a TLVchange and, as a consequence, enters the fast-transmit mode. In the fasttransmit mode, an information exchange occurs immediately between themaster switch and a specific remote switch, which can thus avoidtriggering an unnecessary switch-gone event if the aging out is caused,for example, by dropping related SDPDUs on the wire for unknown reasons.

All network elements implement another timer to remove the switchinformation for a remote switch if the Switch-Info TLV of that remoteswitch has not been received for a specified period (e.g., 120 seconds).When this occurs, a switch-gone event is detected and the MTL of the SDPmodule is notified. When notified by the PSL 26, or other modules, thata switch is gone, the switch information learned for that switch iscleared from the MTL (or marked “unavailable”). For instance, othermodules can signal a path failure (i.e., a path is unhealthy) under thefollowing situations: a link down, packet loss detected by protocolssuch as PSL/EL2T/RPC, etc.

Two parameters for implementing the age-out mechanisms include anSI-AGE-OUT-INTERVAL and a SWITCH-AGE-OUT-INTERVAL. The value held by theSI-AGE-OUT-INTERVAL parameter is the time interval for aging out a SIsequence number of a remote switch in the Switch-Members TLV. Itsdefault value can be, for example, 100 seconds. If the value is setequal to 0, then the aging mechanism is disabled. The value held by theSWITCH-AGE-OUT-INTERVAL parameter is the time interval to age out theswitch information of a remote switch. Its default value can be, forexample, 120 seconds. If the value is set equal to 0, then this agingmechanism is disabled.

In one embodiment, the TSM 200 is not permitted to enter theslow-transmit mode if the only method for detecting a switch-gone eventis by the aging mechanism; this ensures that the aging mechanism usesthe faster timer rate for detecting a switch-gone event.

FIG. 10 shows an embodiment of the RSM 250 for the SDP. The RSM 250controls the processing of SDPDUs received by a network element. The RSM250 includes an idle state 252, a Receive state 254, and a Process state256. The idle state 252 and Receive states 254 are considered stablestates. Three events can trigger a state transition in the RSM 200:start-up, shut-down, and an SDPDU received.

A network element enters the idle state 252 when it initializes (startsup) and when it shuts down. From in the idle state 252, the RSM 200transitions to the Receive state 254. The RSM 250 remains in the Receivestate 254 until a SDPDU is received (or the network element powersdown). In response to receiving an SDPDU, the RSM 250 transitions to theProcess state 256. In the Process state 256, a follower switch processesboth the Switch-Info TLVs received in an SDPDU and the Switch-MembersTLVs received from the master switch. The master switch, in contrast,processes the received Switch-Info TLVs (i.e., not Switch-Members TLVs).

In addition, if a TLV agreed event occurs while in the Process state256, the LOCAL-FAST-XMIT variable is set equal to 0. As previouslydescribed, after the master switch and follower switches have receivedstabilized current information in Switch-Info and Switch-Members TLVs,the RSM 250 triggers the TSM 200 to enter the slow-transmit mode. Themaster switch, however, can trigger the TSM 200 of a network element toenter the fast-transmit mode by setting the corresponding SI sequencenumber of that network element in the Switch-Members TLV to 0.

In addition, in the Process state 256, if the SM.SI_ACKed (i.e., field172 in the SM) is equal to zero, the RSM 250 notifies the TSM 200 tomove to the fast-transmit state 206. The master switch can use thismechanism to cause the network element to return to the fast-transmitmode. The network element also processes the SDPDU packet, with the RSM250 transitioning back to the Receive state 254 upon completion.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method, and computer programproduct. Thus, aspects of the present invention may be embodied entirelyin hardware, entirely in software (including, but not limited to,firmware, program code, resident software, microcode), or in acombination of hardware and software. All such embodiments may generallybe referred to herein as a circuit, a module, or a system. In addition,aspects of the present invention may be in the form of a computerprogram product embodied in one or more computer readable media havingcomputer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readable signalmedium or a computer readable storage medium. A computer readablestorage medium may be, for example, but not limited to, an electronic,magnetic, optical, electromagnetic, infrared, or semiconductor system,apparatus, or device, or any suitable combination of the foregoing. Morespecific examples (a non-exhaustive list) of the computer readablestorage medium would include the following: an electrical connectionhaving one or more wires, a portable computer diskette, a hard disk, arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM or Flash memory), an optical fiber,a portable compact disc read-only memory (CD-ROM), an optical storagedevice, a magnetic storage device, or any suitable combination of theforegoing. In the context of this document, a computer readable storagemedium may be any tangible medium that can contain, or store a programfor use by or in connection with an instruction execution system,apparatus, or device.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including, but not limited to,electro-magnetic, optical, or any suitable combination thereof. Acomputer readable signal medium may be any computer readable medium thatis not a computer readable storage medium and that can communicate,propagate, or transport a program for use by or in connection with aninstruction execution system, apparatus, or device.

Program code embodied on a computer readable medium may be transmittedusing any appropriate medium, including but not limited to wireless,wired, optical fiber cable, radio frequency (RF), etc. or any suitablecombination thereof.

Computer program code for carrying out operations for aspects of thepresent invention may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as JAVA, Smalltalk, C++, and Visual C++ or the like andconventional procedural programming languages, such as the C and Pascalprogramming languages or similar programming languages. The program codemay execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, includinga local area network (LAN) or a wide area network (WAN), or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

The program code may execute entirely on a user's computer, partly onthe user's computer, as a stand-alone software package, partly on theuser's computer and partly on a remote computer or entirely on a remotecomputer or server. Any such remote computer may be connected to theuser's computer through any type of network, including a local areanetwork (LAN) or a wide area network (WAN), or the connection may bemade to an external computer (for example, through the Internet using anInternet Service Provider).

Aspects of the present invention are described with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems) and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer program instructions. These computer program instructions maybe provided to a processor of a general purpose computer, specialpurpose computer, or other programmable data processing apparatus toproduce a machine, such that the instructions, which execute via theprocessor of the computer or other programmable data processingapparatus, create means for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

Aspects of the described invention may be implemented in one or moreintegrated circuit (IC) chips manufactured withsemiconductor-fabrication processes. The maker of the IC chips candistribute them in raw wafer form (on a single wafer with multipleunpackaged chips), as bare die, or in packaged form. When in packagedform, the IC chip is mounted in a single chip package, for example, aplastic carrier with leads affixed to a motherboard or other higherlevel carrier, or in a multichip package, for example, a ceramic carrierhaving surface and/or buried interconnections. The IC chip is thenintegrated with other chips, discrete circuit elements, and/or othersignal processing devices as part of either an intermediate product,such as a motherboard, or of an end product. The end product can be anyproduct that includes IC chips, ranging from electronic gaming systemsand other low-end applications to advanced computer products having adisplay, an input device, and a central processor.

Many modifications and variations will be apparent to those of ordinaryskill in the art without departing from the scope and spirit of theinvention. The embodiments were chosen and described in order to bestexplain the principles of the invention and the practical application,and to enable others of ordinary skill in the art to understand theinvention for various embodiments with various modifications as aresuited to the particular use contemplated.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It is be further understood that the terms “comprises” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed.

While the invention has been shown and described with reference tospecific preferred embodiments, it should be understood by those skilledin the art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the following claims.

What is claimed is:
 1. A method for discovering network elementsassigned to a same group in a distributed fabric system, the methodcomprising: periodically multicasting, by each network element of aplurality of network elements, switch discovery protocol data units(SDPDUs) using one of a plurality of transmission rates, the pluralityof transmission rates including a fast transmission rate and a slowtransmission rate; detecting, by one or more of the network elements,stability in the distributed fabric system; and changing, by the one ormore network elements that detected stability, the transmission ratefrom the fast transmission rate to the slow transmission rate inresponse to the detected stability.
 2. The method of claim 1, whereinthe plurality of network elements includes a master network element, andwherein the stability is detected when information exchanged between themaster network element and the one or more network elements thatdetected the stability has stabilized.
 3. The method of claim 1, whereinthe detected stability is a number of consecutive transmissions of anunchanged SDPDU that exceeds a threshold.
 4. The method of claim 1,further comprising detecting, in response to a received SDPDU, a changeamong the network elements in the same group and to changing thetransmission from the slow transmission rate back to the fasttransmission rate.
 5. The method of claim 1, further comprising electingone of the network elements as a master network element for the group.